The girls, ages 17 and 15, threatened the teen with a knife, kicked him in the groin and dragged him around by his hair, said St. Mary’s County Sheriff’s Office Sgt. Cara Grumbels. They coerced him into walking on a partially frozen pond and then refused to help him out of the frigid water, she said.

Grumbles said the boy got out himself, but the prank could have turned deadly.

“You’re dealing with somebody who doesn’t have the mental capacity of you and I,” she said. “Somebody like that could go into a kiddie pool and may not be able to get themselves out. That’s what’s really kind of disturbing to us, among the other allegations in this case. The whole thing’s just very disturbing.”

She said investigators haven’t found any of the video online.

The 17-year-old, Lauren A. Bush, of Mechanicsville, was charged as an adult Tuesday with first- and second-degree assault, false imprisonment and solicitation for child pornography. She was released on her own recognizance.

The 15-year-old was charged as a juvenile with the same offenses and referred to the state Department of Juvenile Services. The sheriff’s office didn’t release her name.

Grumbles said both girls acknowledged they committed the offenses.

Grumbles said the boy’s parents told police he is autistic and police described him as having “diminished mental capacity.”

She said all three teens attended Chopticon High School in Morganza.

Police learned of what happened from a parent of the younger girl who had seen the video on her daughter’s phone, Grumbles said. The parent showed it to a sheriff’s deputy who works in the school, she said.

Grumbels said there is no evidence other students joined the bullying.

She said more charges are likely as investigators list offenses for each episode that authorities say occurred from early December to early February.